1. Field of the Invention
The present invention relates to a thermoplastic resin composition having excellent thermal stability, which presents excellent surface gloss when formed into a molded product, particularly when injection-molded at a high temperature.
2. Description of the Prior Art
ABS resins have well balanced physical properties such as mechanical strength, moldability and surface gloss, and they are widely used in various fields including automobile parts, parts of electric appliances, and parts of office appliances. Among such uses, there are some wherein particularly good surface gloss is required.
It is known that the surface gloss of a molded product of ABS resin is very much influenced by the particle size of rubber particles contained in the resin. Namely, the smaller the particle size is, the higher the surface gloss of the ABS resin becomes. Further, the surface gloss is also influenced by the dispersed condition of rubber particles. Namely, the better the dispersed condition is, the higher the surface gloss of the resulting ABS resin becomes.
In an ABS resin containing rubber particles having a small particle size, the distance between the rubber particles dispersed in the resin is smaller than that of an ABS resin containing rubber particles having a large particle size, and therefore the frequency of collision of the rubber particles to one another during the melt shearing process is higher. Accordingly, in the case of rubber particles having a low graft rate, agglomeration of the rubber particles is likely to take place due to the collision. Accordingly, the surface gloss of a molded product thereby obtainable tends to be poor, and in an extreme case, silver streaking appears on the surface of the molded product.
Namely, in order to obtain an ABS resin having good surface gloss, it is necessary to employ a rubber having a small particle size and to sufficiently increase the graft rate of the rubber particles. To obtain such an ABS resin, a so-called emulsion graft polymerization method is most suitable wherein emulsion polymerization of monomers to be grafted is conducted in the presence of a rubber latex obtained by emulsion polymerization. However, in an ABS resin obtainable by such emulsion graft polymerization, the emulsifier and a reaction product of the emulsifier with a precipitating agent i.e. an emulsifier reaction product, are likely to remain. Accordingly, undesirable phenomena for molding, such as a deterioration of the thermal stability attributable to the emulsifier and the emulsifier reaction product, are observable.
The deterioration of the surface gloss of the ABS resin is distinctly observed especially in the case of injection molding at a high temperature. Likewise, the deterioration of the thermal stability is remarkable during the high temperature molding. Such deteriorations observed during the high temperature molding of the ABS resin put a substantial limitation to the molding conditions for producing a molded product of ABS resin having a good outer appearance, such being undesirable from the industrial point of view.
It is known to dilute an ABS resin having a high content of a diene rubber produced by emulsion graft polymerization, with an ABS resin having a low content of a diene rubber or an AS resin containing no diene rubber, produced by suspension polymerization or bulk polymerization, to produce an ABS resin wherein the content of the emulsifier reaction product is suppressed. However, when an ABS resin having a high content of a diene rubber is produced by emulsion graft polymerization by using a rubber having a small particle size, and then diluted to obtain an ABS resin having a desired rubber content, the surface gloss of a molded product of the ABS resin thereby obtained will be low, and in an extreme case, silver streaking will be observed on the surface of the molded product.